Theory of Failure for Ductile Materials

(previously posted on alt.mechanical.engineeing)

I've been doing some bolt stress analysis recently and came across the
following issue:

Following the methods taught by Shigley we typically use the maximum
principal stress theory of failure to judge whether or not a bolt will break
when it is being tightened.. The plane stress state in the bolt during
tightening consists of tension and shear. I calculate the max principal
stress and compare it to the ultimate strength of the bolt to determine my
design margin. However, at my company we use a lot of 300 stainless steel
screws that are basically in the annealed state (80ksi ultimate and 30 ksi
yield). In actuality the screws probably get work hardened during heading
and thread rolling but the specs say they don't. So, I have to assume I
have very ductile fasteners.
In reading Shigley he justifies max principal stress by saying it only
applies to high strength bolts (SAE grade 3 or higher?) that behave like
brittle materials (no appreciable yield before fracture).
I went back and re-read the section on theories of failure and, sure
enough, max principal stress is only recommended for brittle materials. It
isn't "safe" for ductile materials because it assumes the material is as
strong in shear as tension. That leaves me with max shear and max distortion
energy (von Mises Hencky). These theories are recommended for ductile
materials with max shear being the most conservative.
My problem is that I want a theory of failure that applies to *fracture*
of ductile materials. Max shear and max distortion are only recommended for
determining the onset of yield. I don't care if my 300 stainless screws
undergo a little plastic deformation (strain hardening?) as long as they
dont break off.

Any suggestions?

Al


.

Relevant Pages

  • Re: Shear Strength
    ... Materials Failure Analysis", Pergamon Press, Oxford 1993 under the ... stress in shear is approximately 5/8 of the tensile strength. ... called 'complex stresses' the principle stresses set up complementary ... stress always resulted in a stress of a given degree of the applied stress. ...
    (sci.engr.metallurgy)
  • Re: Theory of Failure for Ductile Materials
    ... I've been doing some bolt stress analysis recently and came across the ... In reading Shigley he justifies max principal stress by saying it only ... max principal stress is only recommended for brittle materials. ...
    (sci.engr.mech)
  • Re: Brakes: changing from nutted to recessed?
    ... for any responsible cut thread bolt - it's cheaper/weaker material because the better properties of the rolled thread allow it to be used. ... weaker materials that are deemed sufficient, ... Stress on a brake bolt is at the forward end in bending for which its ...
    (rec.bicycles.tech)
  • Re: Reaming
    ... Note that stress is the distribution of force in a material. ... developing much friction away from the bolt holes. ... Perhaps one should also check the bolt tension frequently. ... because the cyclic portion of the load is transfered via friction. ...
    (rec.aviation.homebuilt)
  • Re: Theory of Failure for Ductile Materials
    ... I've been doing some bolt stress analysis recently and came across the ... In reading Shigley he justifies max principal stress by saying it only ... max principal stress is only recommended for brittle materials. ...
    (sci.engr.mech)